Hi merli firstly you will not fit the 28s because they will expand too much and hit the guards and you won't fit the 26x10s unless you fit solid bars instead of suspension as it was our first run with the 26x10s cought the side skirts and ripped them off and put a big dent in the tyres so i suggest the 26x8.5s i have run a handfull of 9sec passes at over 140mph on that size with a standard diff, drive & tail shaft as for the gearbox i cant help you there other than it's cheaper to replace a cluch than a gearbox so just flair the cluch a bit, as for the wheels i dont know off hand what the offset is but i do remember it was the most furtherists offset out that centre line do on the convos ring shane if you get stuck, hope this is of help.
brett.